Electroluminescent device with light emitting aromatic, hydrocarbon material

ABSTRACT

A thin body of active material in the form of anthracene or naphthalene generates light when a current is passed through it by the injection of electrons from an electrode containing negative ions of a similar material e.g. anthracene, naphthalene or tetracene) to a base electrode to which an external positive potential relative to the electron-injecting electrode is applied. The base electrode may be solid and transparent for transmission of light generated in the active material or at its interface with the base electrode. This device also acts as a rectifier, since virtually no current will flow if the externally applied potential is reversed.

Uniteri States Patent [72] Inventors Dlgby F. Williams 2,938,136 5/1960Fischer 313/108 Ottawa; 3,172,862 3/1965 Gurnee et a1. 313/108 UX MartinSchadt, Vanier City, Ontario, both 3,382,394 5/1968 Mehl 313/108 ofCanada 3,391,068 7/1968 McKay 313/108 X [21] PP 871,347 3,530,325 9/1970Mehl et al 313/108 Filed Oct-28,1969 OTHER REFERENCES 22:5 gzngg gzi andDevelopment H lnokuchi et al., The Photovoltaic Behaviors of Aro- Limuedmatlc Hydrocarbons, Electrical Conductivity in Orgamc Ottawa Ontario,Canada Solids, pp. 69- 75, 1961.

Primary Examiner-Roy Lake Assistant Examiner-Palmer C DemeoELECTROLUMINESCENT DEVICE WITH LIGHT Attorney-Stevens, Davis, Miller &Mosher EMITTING AROMATllC, HYDROCARBON MATERIAL 6 Claims, 1 Drawing Fig.

ABSTRACT: A thin body of actlve material In the form of [52] U.S.Cl313/108 A, anthracene or naphthalene generaes li ht when a curl-em i317/235 AF passed through it by the injection of electrons from an elec-[51] Int. Cl. ..H05b33/02, "ode containing negative ions f a Similarmateria] 33/26 anthracene, naphthalene or tetracene) to a base electrodeto [50] Field of Search 3 l 5/ 108 R, which an external positivepotential rdative to the electromin. 108 358; 317/235 jecting electrodeis applied. The base electrode may be solid and trans arent fortransmission of light generated in the ac- [56] Referencw Cited tivematei'ial or at its interface with the base electrode. This UNITEDSTATES PATENTS device also acts as a rectifier, since virtually nocurrent will 2,834,903 5/ 1958 Roberts 313/108 flow if the externallyapplied potential is reversed.

A/E4A 7/ v5 fA/C'A P5 04 A 77A/6' WA X//4 4 5: 72005 /2 l\\\\\\\\%&\\\\\\Y 6 RETAIN/N4 n (Yo/V006 n/vg 61/1 55 1 ACTIVE MA nae/ 4z 1/ 64 7mm dfA/E) ELECTROLUMINESCENT DEVICE WITH LIGHT EMITTINGAROMATIC, HYDROCARBON MATERIAL This invention relates toelectroluminescent devices. Electroluminescence over a considerablewavelength region is possible using inorganic semiconductor systems.However, at least as versatile in this respect are system using organicmaterials as the active elements. Such organic materials are normallyhighly electrically nonconducting, however, so that beforeelectroluminescence can be achieved it is necessary to find the correctconditions for injecting electric charges (electrons or holes) into thematerials.

While, as indicated below, the present invention is not restricted inits broad scope to the injection of charges into any one or twoparticular materials, it will be convenient to take as typical materialsanthracene and naphthalene, whether in the form of single crystals or aspowder or in a matrix.

lnjection of electric charge carriers into such materials has beenachieved using liquid electrodes. See for example H. Kallman et al., J.Chem. Phys, 32,300 1960); M. Pope et al., J. Chem. Phys, Vol. 36, No. 9,May 1, I962, pages 2486 et seq.; and W. Helfrich et al. U.S. Pat. No.3,457,153 issued July 22, 1969. Only in the latter reference weresufficient currents reported to generate any observable luminescence.Emission from an anthracene crystal occurred primarily in the bluespectral region, and is believed to have been generated by the directrecombination in the body of the crystal of electrons and holes injectedfrom the opposite faces of the crystal by the respective liquidelectrodes.

These liquid electrodes are, however, opaque and it has been necessaryto remove the light from the system through the edges of the activecrystals. This is inconvenient and relatively inefficient, especiallysince for best performance the crystal will normally be made as a verythin wafer, the liquid electrodes obscuring most or all of the flatfaces. Also only relatively short term stability was available due todissolving the active material in the electrode solutions.

The principal objects of the present invention are to provide a methodof injecting carriers into such materials by means of electrodes atleast one of which is a transparent solid through which the lightgenerated in the active material can readily emerge, and to givelong-term stability to the system.

Another object is to obtain more light than has previously been possiblewith devices of this type of comparable size and under comparableoperating conditions.

Yet another object is to produce a device that will act as a rectifieras well as to produce light.

The present invention consists of an electroluminescent devicecomprising:

a. a thin body of light-generating, aromatic, electricallynonconducting, hydrocarbon material,

b. a first electrode having an electrically conducting surfacecontacting a first surface of said light-generating material, and

c. a second electrode contacting a second surface of saidlight-generating material, said second electrode containing negativeions of an aromatic hydrocarbon material capable of injecting electronsinto said second surface upon the application of a negative potential tosaid second electrode relative to a potential applied to saidelectrically conducting surface of the first electrode;

d. at least one of the said electrodes being in solid phase andtransparent for the transmission therethrough of iight generated in saidlight-generating material by recombination of said injected electronswith holes.

The single figure of the accompanying drawing shows diagrammatically andby way of example a cross section of one device constructed inaccordance with the present invention.

This device comprises a base of conducting glass; then a layer 11 of theactive material; then a layer of negative electrode material 12 heldwithin a glass-retaining ring 13, if necessary; and finally a body ofencapsulating wax l4. Leads 15 and 16 connect to external terminals towhich either relative positive or negative direct voltage is applied asillustrated.

That is the glass 10 will always be positive relatively to the electrode12, the potential of these parts relative to ground being unimportant.

The base 10 is of glass, at least the upper surface of which has beenmade conducting by one of the known surface treatment methods, to form afirst electrode. Alternatively, this conducting glass surface electrodecan be replaced by silver or gold paste. However, since these materialsare opaque, they would have to be constructed with windows to permit thelight emission.

The light-generating active material ill should be kept as thin aspossible, since the light generated is proportional to the current, andthe current is proportional to the square of the voltage and inverselyas the cube of the thickness of the material. A thickness of the orderof l to 200 microns is preferred. This active material may take the formof a crystal or may be in powder or other form. For example, thefollowing materials have been used satisfactorily as the activematerial:

a. Anthracene sublimation flakes.

b. Anthracene melt grown single crystals.

c. Anthracene powder.

d. Anthracene powder in a matrix of methyl methoxylate plastic.

e. Naphthalene sublimation flakes.

The chemical form should be of high purity for maximum emissionintensity, and, although anthrancene and naphthalene have been taken asthe most convenient and readily available substances, the activematerial can theoretically be any aromatic hydrocarbon in a relativelypure chemical form. In practice, however, only those materials having ahigh quantum efficiency of fluorescence can be used with advantage.

The electrode material 12 will preferably consist of solid material madeby evaporation from a solution containing negative ions of a hydrocarbonsimilar to that used as the active material. For example, if the activematerial lll is anthracene, the electrode material 12 can be theevaporation product of anthracene dissolved in a solvent such as willproduce stable negative anthracene ions, e.g. tetrahydrofuran,dimethoxyethane, dimethyl sulfoxide or dimethyl formamide. The materialcan be prepared by interacting the solution with metallic sodium orother alkali metal, care being taken to keep the material under an inertatmosphere such as nitrogen during production. Direct production of thiselectrode material 12 by reduction of the active material 11 at itssurface is also possible by sublimation of the reductant, e.g. sodiummetal.

If the active material 11 is naphthalene, the electrode material i2 canlikewise be a similar evaporated product of a negative ion producingsolution of naphthalene. While it is convenient to use the samehydrocarbon for both materials this is not essential. For example, theelectrode material 12 can be naphthalene or tetracene while the activematerial lll is anthracene.

The retaining ring 113 is solely physical in function and will beunnecessary if the electrode material 12 is deposited upon thelight-emitting material surface by other techniques, e.g. sublimation.The wax M can be replaced by any other inert substance or means forpreventing access of air.

Upon application of the relative voltages shown, light is emitteddownwardly either through the: glass base or upwardly through theelectrode material 12 and wax 14, if these are sufficiently transparent,or in both directions. There will also be emission from the edges of theactive material lll unless this is deliberately discouraged by someopaque material. The wavelength of the emitted light may be varied bythe choice of hydrocarbon for the active material. Naphthalene will emitin the ultraviolet range mainly from approximately 3500 to 4000A, andanthracene in the blue range mainly from approximately 4100 to 4500A.Wavelength changes can be achieved by doping. For example, anthracenedoped with one part per million of tetracene gives a green-blue emissionat approximately 48005000A. lt has also been found that color variationscan be obtained by varying the voltage or the dopant concentration.

If the externally applied potential is reserved, virtually no currentflows, so that the device acts as a rectifier. The forward and reversecurrent ratio is of the order of Under the condition of flow of injectedelectrons from the second electrode 12 to the first electrode 10 thelatter must be able to supply sufficient holes to recombine with asufficient number of electrons to produce the required intensity ofelectroluminescence. Many electrode materials satisfy this criteria, forexample conducting glass, silver paste and gold paste. If there issubstituted a solid substance capable of injecting holes into the lowersurface of the active material 11, for example anthracene positive ionsor naphthalene positive ions, a current will flow, light will begenerated, and also a rectifying action will be observed. The positiveion electrodes, however, are opaque and only permit light emitted fromthe sides of the device to be observed.

We claim:

I. An electroluminescent device comprising:

a. a thin body of light-generating, aromatic, electricallynonconducting, hydrocarbon material,

b. a first solid phase electrode having an electrically con ductingsurface contacting a first surface of said light generating material,

c. a second solid phase electrode contacting a second surface of saidlight-generating material, said second electrode containing negativeions of an aromatic hydrocarbon material capable of injecting electronsinto said second surface upon the application of a negative potential tosaid second electrode relative to a potential applied to saidelectrically conducting surface of the first electrode, and

d. at least one of said electrodes being transparent for thetransmission therethrough of light generated in said lightgeneratingmaterial by recombination of said injected electrons with holes.

2. A device according to claim I, wherein said light-generating materialis anthracene and said second electrode includes negative anthraceneions.

3. A device according to claim 1, wherein said light-generating materialnaphthalene and said second electrode includes negative naphthaleneions.

4. A device according to claim 1, wherein said light-generating materialis anthracene and said second electrode includes negative naphthaleneions.

5. A device according to claim 1, wherein said light-generating materialis anthracene and said second electrode includes tetracene ions.

6. A device according to claim 1, wherein said first electrode is formedof electrically conducting glass.

1. An electroluminescent device comprising: a. a thin body oflight-generating, aromatic, electrically nonconducting, hydrocarbonmaterial, b. a first solid phase electrode having an electricallyconducting surface contacting a first surface of said light-generatingmaterial, c. a second solid phase electrode contacting a second surfaceof said light-generating material, said second electrode containingnegative ions of an aromatic hydrocarbon material capable of injectingelectrons into said second surface upon the application of a negativepotential to said second electrode relative to a potential applied tosaid electrically conducting surface of the first electrode, and d. atleast one of said electrodes being transparent for the transmissiontherethrough of light generated in said light-generating material byrecombination of said injected electrons with holes.
 2. A deviceaccording to claim 1, wherein said light-generating material isanthracene and said second electrode includes negative anthracene ions.3. A device according to claim 1, wherein said light-generating materialnaphthalene and saId second electrode includes negative naphthaleneions.
 4. A device according to claim 1, wherein said light-generatingmaterial is anthracene and said second electrode includes negativenaphthalene ions.
 5. A device according to claim 1, wherein saidlight-generating material is anthracene and said second electrodeincludes tetracene ions.
 6. A device according to claim 1, wherein saidfirst electrode is formed of electrically conducting glass.